Last week, dozens of American-based media and Internet companies went dark for 500 million citizens of the European Union. The culprit? The E.U.’s General Data Protection Regulation (known as G.D.P.R.), which forces companies to adhere to certain guidelines when it comes to the use and storage of people’s data. The law is already causing panic in Europe: in addition to the blackout, tech giants have been hit with multi-billion-dollar complaints filed by European privacy advocates, and programmatic ad buying has plummeted. But the E.U. is on the verge of an even more dire privacy crisis, which could provide a chilling preview of Silicon Valley’s fate should the U.S. choose to follow in its footsteps.